Field of the Disclosure
The present disclosure relates to an organic light emitting display device, and more particularly, to an organic light emitting display device with reduced operating voltage and improved efficiency.
Discussion of the Related Art
Image displays used for displaying a variety of information on the screen are one of the core technologies of the information and communication era. Such image displays have been developed to be thinner, lighter, more portable, and to have high performance. With the development of the information society, various demands for display devices are on the rise. To meet these demands, research on panel displays such as liquid crystal displays (LCD), plasma display panels (PDP), electroluminescent displays (ELD), field emission displays (FED), organic light emitting diodes (OLED), etc. is actively under way.
Among these types of displays, the organic light emitting display devices are a type of devices that, when the charges are injected into an organic emissive layer formed between an anode and a cathode, the emission of light due to the formation of electron-hole pairs takes place and extinguishes. The organic light emitting display devices are advantageous in that they can be fabricated on a flexible transparent substrate such as plastic substrate, can be operated at a relatively low voltage, have less power consumption, and deliver vivid color reproduction, as compared with the plasma display panels or the inorganic light emitting displays. Particularly, the white OLED devices are used for various purposes in lighting, thin light sources, backlights for liquid crystal displays, or full-color displays employing color filters.
An organic light emitting display device has a lamination structure of an anode, a hole injection layer, a hole transport layer, an light emitting layer, an electron transport layer, an electron injection layer, and a cathode, and the hole injection layer and the electron injection layer are used to facilitate charge injection. A P-type hole injection layer, which is a type of hole injection layer, is involved in the generation, injection, and transport of holes. The P-type hole injection layer is a layer formed of a single P-type material, or includes a host and a P-type material therein. The host serves to inject holes from the anode into the light emitting layer through the HOMO (highest occupied molecular orbital) energy level, and is a material commonly used as the hole injection layer. The P-type dopant is a material that has a strong electron-attracting substituent and attracts electrons from the LUMO (lowest unoccupied molecular orbital) energy level of the hole transport layer adjacent to the P-type hole injection layer to the HOMO energy level of the P-type dopant. The P-type hole injection layer with a strong electron-attracting substituent forms a hole transport path by accepting electrons from the HOMO energy level of the host or the HOMO energy level of the hole injection layer or hole transport layer to the LUMO energy level of the P-type hole injection layer. After all, the LUMO energy level of the P-type hole injection layer and the HOMO energy level of the hole transport layer adjacent to the P-type hole injection layer or the host may be similar in energy level to enable efficient hole generation, so P-type hole injecting materials having a strong electron-attracting substituent are needed.
However, the P-type hole injecting materials are not easy to synthesize due to their strong electron-attracting substituent, and have problems with thermal stability and deposition stability. Particularly, F4-TCNQ, one of the P-type hole injecting materials, sublimes easily, which affects the contamination of deposition sources, the performance reproducibility and thermal stability of devices during device fabrication. Moreover, it is not easy to develop P-type hole injecting materials whose LUMO energy level is similar to the HOMO energy level of the host or the HOMO energy level of the hole transport layer. In order to make the LUMO energy level similar to the HOMO energy level, it is necessary to introduce a strong electron-attracting substituent into the P-type hole injecting material. However, the stronger the electron-attracting substituent is, the harder it is to improve the purity of the material, making the synthesis of the material difficult. Besides, it is necessary that the strong electron-attracting substituent does not absorb visible light, which makes the development of P-type hole injecting materials difficult.